Manipuilate joins dynamically.

Question

Hi All,
I would need an help on the query.
I have a table x and column a, b,c, y and z in them.I am joining this table with another table c
IF the column y is not NULL and if z='M' THEN I have to join table c with d
IF the column y is not NULL and if z='N' THEN I have to join table c with e
IF the column y is not NULL and if z='O' THEN I have to join table c with f
Can anubody help me in how to build this join in SQL level???
Thanks in advance.

Answer 1

Generally speaking if you can use UNION(ALL)s for a table(or a view), it is easy to rewrite it with WHERE ... OR ... OR ...
For example:
SELECT select-list
  FROM Table_x
WHERE some_predicates
UNION
SELECT select-list
  FROM Table_x
WHERE aother_predicates
UNION
SELECT select-list
  FROM Table_x
WHERE somemore_predicates
;It would be able to rewrite to:
SELECT select-list
  FROM Table_x
WHERE some_predicates
    OR aother_predicates
    OR somemore_predicates
;Returning to your original problem, my question is that with what column in table x be joined a column(d,e or f) of table c?
Although I assumed temporarily to join table c with column y in table x,
it is easy to change joined column in table x.
I don't know the performance of this, but I thought that it would work:
SELECT *.x, *.c
  FROM table_x x
       LEFT OUTER JOIN
/*  or INNER JOIN      */
       table_c c
         ON  y IS NOT NULL
         AND
           ( z = 'M' AND c.d = x.y
             OR
             z = 'N' AND c.e = x.y
             OR
             z = 'O' AND c.f = x.y
;